79 

 TABLES 52-57.— THE BLACKBODY AND ITS RADIANT ENERGY 



TABLE 52.— SYMBOLS AND DEFINING EXPRESSIONS FOR RADIANT 



ENERGY »* 



Radiant energy is energy traveling in the form of electromagnetic 

 in units of energy such as ergs, joules, calories, and kilowatt hours, 

 and abbreviations used in discussing radiant energy are as follows : 



waves. It is measured 

 Some units, symbols, 



Designation 



Radiant energy 



Symbol and 



defining 

 expression 



u 



Spectral radiant energy. 



Radiant energy density. u = 



Radiant flux <J>(P) = 



Radiant flux density. 



Radiant intensity of 

 source 



W = 



J = 



Spectral radiant intensity A = 



Radiant flux density of 

 a source per unit solid 

 angle B,(N) = 



Radiant intensity of a 

 source per unit area. . 



Radiant flux per unit 

 area 



B = 



watt/(steradian cm*) 

 watt/(steradian cm 1 ) 



E = 



dA 



Steradiancy * 



Steradiancy * 

 Irradiancy 



The standard radiator is the blackbody, which may be defined as a body that absorbs 

 all the radiation that falls upon it, i.e., it neither reflects nor transmits any of the incident 

 radiation. From this simple definition and some very plausible assumptions it can be shown 

 that the blackbody radiates more energy than any other temperature radiator when both 

 are at the same temperature. The total amount of energy (i.e., for all wavelengths) 

 radiated by a blackbody depends upon the temperature raised to the fourth power and a 

 constant a that had to be measured: 



If a blackbody is radiating to another blackbody it will at the same time receive radiation 

 from the second blackbody and, under the proper geometrical conditions, the net radiation 

 lost by the first blackbody is 



W = a(Tx'— 7V) 



The spectral distribution of this radiation is given by the Planck equation : 



/ A =tftW[exp (ft/XT) — 1] t 



For values of the product XT less than 3000m deg, the Wien equation 



A = CiX- 6 /[exp( < r 1 /\r)] 



gives values that are correct to better than 1 percent. 



The values of a number of the radiation constants have been selected from Table 26 and 

 are given in Table 53. All the blackbody calculations given were made with these constants. 

 Some calculated results ** for the total radiation W for a series of temperatures and of A 

 for a range of temperatures and for wavelengths have been calculated and are given in 

 Tables 54-56. 



n » Rev. Sci. Instr., vol. 7, p. 322, 1936. * These terms apply only to a source. The term "radiance" 



is not recommended as a substitute for radiant flux; however, if a single term is desired to express the 

 radiant flux from a source, the word "radiance" is suggested as the most logical. t See footnote 5a, 



P. 7. 



84 For a more extensive list of values of 7 X reference should be made to two papers by Parry Moon : 

 Journ. Math, and Phys., vol. 16, p. 133, 1937; Publ. Electr. Eng., Massachusetts Institute of Tech- 

 nology, 1947. 



SMITHSONIAN PHYSICAL TABLES 



